Coverage of the Studies

Currently, there are several known models for canopy rainfall retention during rainfall (excluding stem flow which is no more than 10% of the total interception). The most common models are the ones of Horton, Merrian, Jackson, Gash, Fan (Gash et al., 1995; Pypker et al., 2012; Klaassen et al., 1998; Link et al., 2004; Herbst et al., 2008; Muzylo et al., 2009).
In the present study, the authors have carried out a set of experiments on artificial creation of droplets on leaf surface of individual branches, determination of the maximum mass of water retained and calculation of the leaf surface area of the analyzed samples, determination of the maximum sizes of droplets and their edge angles and determination of empirical dependencies of the leaf surface area of individual tree species on the maximum mass of water retained.
The Gash and Fan models are considered to be the most modern methods for estimating canopy rainfall interception. With an actual interception of a single rainfall event of 33.2% of the total amount of precipitation, the estimated interception ratio from the Gash model was 35.9% and from the Fan model 53.6%.
The common feature of all models is that they rely on empirical observations of net rainfall rather than on physical modeling of moisture retention on leaf surfaces. None of the models uses the objective values of the droplet mass on the lamina and the leaf surface area, which the authors of this study have managed to obtain by artificial sprinkling of the branches and weighing of the branches, followed by leaf area estimation.
The formation of the peak rainfall flood discharges to small watercourses of the forest zone is determined by rainfall input to the watershed surface, various losses, slope runoff formation and its transformation into runoff hydrograph at the outlet (Golubtsov V.V., 2010). The most significant initial losses are rainfall interception by canopies of coniferous and deciduous trees, as well as rainfall interception loss by evaporation off the leaf surface of the canopies.
In order to estimate the spatial rainfall distribution intercepted by the canopies of coniferous tree stands, a set of experimental, cartographic and laboratory studies as well as meteorological observations on the estimation of water retention on the phytomass of forest-forming species of the Middle Urals have been performed. The forest watershed of the Reshetka River which is near the village Novoalekseevskoe (watershed area F=32.0 km2) was chosen as an experimental site. The watershed located in the Sverdlovsk Region is 31 km west of the Ekaterinburg weather station and is equipped with the Roshydromet observation station that has been operating since 1946 up to the present day (Fig. 1). Main hydrographic characteristics of the watershed: weighted-average watershed slope 33.1%; forest cover 82% (72% accounts for pine; 8% - birch; 2% - other tree species (spruce, aspen, linden, etc.); lake percentage- 0%; swampiness - 0%; average height - 22 m. Tree species of the studied area belong to the spring-summer-autumn-green phenotype with the period of winter-spring dormancy. Vegetation lasts from the second decade of May to the first decade of October. The green mass of deciduous trees reaches its maximum growth by the end of the first decade of July. During this time the leaf surface area is maximal and this period corresponds to the period of maximum rainfall frequency. Within the reference watershed, a set of field and desktop research was carried out to clarify the forest stand characteristics (height and species composition) and hydrographic works. Methods of estimating precipitation losses from the tree canopies during the period of rainfalls of various intensities were developed and tested.
The liquid of (rainfall or condensation) is retained on the tree leaf surface in droplet form and the retention is provided by surface energy at the phase boundaries between ”solid” and ”gas” and ”liquid”.
Due to the fact that most of the liquid precipitation is kept by coniferous and leaf surfaces, the first task to be solved is to determine the biometric indices of trees:
- leaf mass per 1 ha (F1, t/ha);
- leaf area index (ratio of leaf area to unit ground surface area) (LAI, ha/ha);
- leaf area (total leaf or needle surface area per unit ground area) (LA, ha);
- number and average size of leaves and needles.
The bonitet class of forests, trunk timber reserves and the degree of canopy cover are taken into account.
When estimating the average size of needles and leaves, the approach proposed by A.I. Utkin and others was used (A.I. Utkin et al., 2008). It involves the formation of representative samples of needles and leaves (100 units), and their measurements by automated means (scanning of images and subsequent measurements of area, length, width by AutoCAD). Based on the measurements, statistical parameters (average, the coefficient of variation Cv, the coefficient of asymmetry Cs) of needle and leave size of the analyzed species were determined (Table 2).
The phytomass values of leaves and needles are determined on the basis of growth chart and tables of biological productivity of tree stands (A.Z. Shvidenko et al., 2008), the results of field-forest studies on the surface areas of forest plants obtained with the use of experimental data (A.I. Utkin et al., 2008), as well as space images and large-scale maps. A bonitet class is determined on the basis of field studies (data on height and age of stand) using M.M. Orlov’s tables.
Using the databases and empirical dependencies established by A.I. Utkin and others (Table 2), LAI was determined on the basis of the phytomass value (F1, t/ha) of stand species as LAI = f(F1).
The total leaf surface area (LA, ha) is defined as:
, (1)
where S is the area of the forest plot (ha), k is the stand density (in unit fractions).
The stand density and its bonitet on the model watershed of the Reshetka River are determined on the basis of field survey, using the tables of M.M. Orlov.
The phytomass value (F1, t/ha) for watershed areas with different species composition has been determined depending on their age and a bonitet class: M.M. Orlov’s tables have been put into analytical form, and the estimation of forest age of a certain bonitet has been made on the basis of stand height data (Fig. 2).
According to the fieldwork materials, the prevailing bonitet class for the model watershed is II. On the basis of works (by Shvidenko A.Z. et al., 2008), the relationship between the phytomass value (green mass and the mass of branches) (F1, t/ha) and the age of tree stands of different species composition having the bonitet class II was determined in the analytical form. In the watershed area of the Reshetka River pine stands prevail, the share of spruce is insignificant (not more than 10% of the total area of coniferous plantations). Among deciduous species, birch and aspen prevail. The forests with the share of deciduous or coniferous trees not less than 25% are considered as a mix of deciduous or coniferous forests. On the basis of these ratios, the dependency curves of phytomass on the age of tree stands have been estimated (Fig. 3).
As can be seen, for the considered watershed under conditions of rather homogeneous species composition of tree stands, the dependencies of F1 values on the forest age are very similar and can be described with a single equation of the form:
F1=11.7-ln(T) - 22.8. (2)
Taking into account the above mentioned dependencies of tree height on age, the estimating equation for F1 for bonitet class II is:
(3)